This invention relates to processes for the production of polyesters containing 1,4-cyclohexanedimethanol. More particularly, this invention relates to such processes wherein the dicarboxylic acid component of the polyester may be added in its acid form rather than its diester form at ambient conditions.
Polyesters are widely used as extrusion and injection molding resins for applications such as fibers, films, sheeting, automotive parts, and food and beverage containers. Polyesters useful for such applications include those containing a glycol component of 1,4-cyclohexanedimethanol (CHDM) and a dicarboxylic acid component such as terephthalic acid and/or isophthalic acid. In manufacturing these polyesters, handling issues arise with respect to supplying these monomers to the reactor since CHDM, terephthalic acid and isophthalic acid are all solid at ambient conditions. The handling of these monomers is of significant concern for continuous processes in which bulk handling and precise metering of solids is extremely difficult.
CHDM, which is a waxy solid at ambient conditions, is typically heated to above its melting point to facilitate a molten transfer of the CHDM to the reactor. However, maintaining CHDM at temperatures above its melting point for long periods of time is not desirable because of increased rates of degradation and the increased energy costs of maintaining a heated feed.
The dicarboxylic acids are typically supplied as a fluid in the form of their lower alkyl ester, such as dimethyl terephthalate or dimethyl isophthalate. The use of the lower alkyl esters of these acids in the polymer manufacturing process developed because of early difficlties in purifying terephthalic and isophthalic acids. However, technology for purification has evolved. Now purified dicarboxylic acids are commonly available for use in the polyester manufacturing process. Because of handling rather than purity, the dicarboxylic acids are often times converted to their ester derivatives prior to being supplied to the reactor. The esters can be melted to form a liquid and thus more precisely metered to the reaction process. This conversion requires additional processes steps resulting in unnecessary costs.
When the dicarboxylic acid, rather than its ester derivative, is used to supply the acid moiety of the polyester, a blend of glycol and dicarboxylic acid in the form of a paste has been used to introduce these raw materials to a batch reactor or the first stage reactor in a continuous process. This process is reasonably effective when the major glycol component is ethylene glycol, which is a liquid at ambient conditions. However, problems frequently arise in feeding such pastes to a reactor when glycols that are solids at ambient conditions, such as neopentyl glycol or CHDM, are used. The glycol and dicarboxylic acid must be heated to sufficient temperature to form a molten paste. Many dicarboxylic acids, especially terephthalic acid and isophthalic acid, degrade upon heating with increasing degradation as temperatures approach the melt temperatures of the dicarboxylic acids. In some cases a mixture of the purified dicarboxylic acid and a dimethyl ester of the same or a different dicarboxylic acid is supplied to the reactor. However, this process does not eliminate the cost of obtaining the ester derivative.
Thus, there exists a need in the art for a process to supply glycols and dicarboxylic acids that are solids at room temperature to a reactor without degradation of the glycols or dicarboxylic acids. Accordingly, it is to the provision of such processes that the present invention is primarily directed.
In a process for producing a polyester containing 1,4-cyclohexanedimethanol (CHDM) as one of its glycol components, an aqueous or a methanolic slurry comprising CHDM and a dicarboxylic acid is prepared. The slurry is maintained at a temperature below the melting point of CHDM. The slurry is then fed into a reactor. The slurry is esterified at sufficient temperatures and pressures, and optionally in the presence of a suitable catalyst, to effect esterification. A prepolymer is formed. The prepolymer is then polycondensed at sufficient temperatures and pressures in the presence of a suitable catalyst to effect polycondensation to form a polyester.